Active steering system

ABSTRACT

A method for operating an active steering system of a vehicle, in which a ratio between a wheel steer angle and a steering wheel angle specified by the driver can be modified by superimposition of a superimposition angle. The method comprises detecting an offset between a requested motor angle and an actual motor angle, and reducing the offset between the requested motor angle and the actual motor angle using a reduction rate that is selected as a function of at least one input variable specified by the driver.

This patent application claims priority to German Patent Application No.DE 102009002743.2, filed Apr. 30, 2009.

INTRODUCTION

The present teachings relate to an active steering system and to amethod for operating an active steering system.

SUMMARY

The present teachings provide a method for operating an active steeringsystem in a vehicle, in which a ratio between a steering wheel angleselected by the driver and a wheel steer angle can be modified by theactive steering system by superimposing a superimposition angle. Tomodify the ratio between the steering wheel angle and the wheel steerangle, the active steering system detects an offset angle between arequested motor angle and an actual motor angle, and reduces the offsetat a reduction rate selected as a function of at least one variableselected by a vehicle operator. The variable can comprise, for example,a steering wheel speed, a rack speed, an offset reduction speed, or aroad wheel speed.

A method according to the present teachings can provide a gradual,gentle elimination or compensation of a deviation (or offset angle)between an actual motor angle and a desired motor angle or a setpointvalue for the motor angle. The offset angle (defined herein as the anglebetween the actual motor angle and the desired motor angle) can beeliminated using the present teachings without the elimination beingperceptible to the driver. In accordance with certain embodiments of thepresent teachings, an offset angle of about 275° can be eliminatedwithout the elimination being perceptible to the driver.

According to various embodiments of the present teachings, eliminationof the offset angle, referred to below as “offset compensation,” can becarried out in a synchronized manner while the vehicle is being steeredby the driver.

According to certain embodiments, input signals that are used forreducing or eliminating the offset angle can comprise, for example, oneor more of the steering wheel speed, the wheel steer angle, and/or thevehicle speed. In certain embodiments, vehicle speed can be utilized tolimit the offset compensation reduction rate, and to determine whetherto do offset compensation, and a rate of offset compensation can becomea function of steering wheel angle and steering wheel speed. A methodaccording to the present teachings takes into account the way the driveris steering the vehicle (rapid steering, steering to the right or to theleft, steering back to the central “straight ahead” position, etc.)

In an exemplary embodiment, for a fixed steering wheel angle (e.g.,90°), when a driver switches from a comfort mode (having, for example, asteering gear ratio of 16) to sporty mode (having, for example, asteering gear ratio of 12), the road wheel angle should change fromabout 90°/16° to 90°/12°. Offsets can also occur during vehicle startup.An algorithm of the active steering system in accordance with thepresent teachings does not allow the motor of the active steering systemto compensate for (e.g., reduce) the offset angle value immediately. Onemethod for compensation can comprise the following: When a driver steersaway from center (the center being straight ahead), a motor of theactive steering system can start to add extra angle in the samedirection so that the vehicle wheels are rotating faster than expectedbut not by an amount perceived by driver. Alternatively or additionally,when a driver steers toward the center, the wheel can move slower thanexpected to reduce the offset.

In accordance with various embodiments, offset angle detection andcompensation are only carried out if there is a request for detection ofthe offset angle. If an offset angle exists but there is no request fordetection of the offset angle, an unmodified setpoint value is outputfor the motor angle.

The present teachings provide a method for operating an active steeringsystem of a vehicle, in which a ratio between a wheel steer angle and asteering wheel angle specified by the driver can be modified bysuperimposition of a superimposition angle. The method comprisesdetecting an offset between a requested motor angle and an actual motorangle, and reducing the offset between the requested motor angle and theactual motor angle using a reduction rate that is selected as a functionof at least one input variable specified by the driver.

The at least one input variable specified by the driver can comprise oneor more of a steering wheel speed and a wheel steer angle, and reducingthe offset can comprise reducing the offset in a manner that issynchronized with a steering operation of the driver. Thesuperimposition angle can be provided by a motor. Detecting the offsetand reducing the offset can occur only upon an external request fordetection of the offset. In the absence of an external request fordetection of the offset, a demanded superimposition angle can beselected as a setpoint value for the superimposition angle.

The present teachings also provide an active steering system for avehicle, comprising: (1) a steering wheel configured to allow a driverto steer the vehicle by changing a steering wheel angle, which isconfigured to change a wheel steer angle of at least one wheel of thevehicle; and (2) a motor receiving a requested motor angle andoutputting an actual motor angle to produce a superimposition angle thatmodifies a steering ratio between the steering wheel angle and the wheelsteer angle. The system can be configured to reduce an offset between arequested motor angle and an actual motor angle as a function of atleast one of a steering wheel speed and the wheel steer angle.

The present teachings also provide an active steering system for avehicle, comprising: (1) a steering wheel and a steering wheel columnconfigured to allow a driver to steer the vehicle; (2) a harmonic driveoperatively connected to the steering wheel column; and (3) a motorreceiving a requested offset angle and producing an actual offset angle.The motor can be configured to drive the harmonic drive to produce asuperimposition angle that modifies a ratio between a steering wheelangle selected by the driver and a wheel steer angle of at least onewheel of the vehicle to reduce an offset between the requested motorangle and the actual motor angle.

The motor may not reduce the offset immediately. A motor angle can beproduced by the motor and superimposed on a driver steering angle viathe harmonic drive. The motor can reduce the offset when the driversteers away from center in a first direction by adding a predeterminedamount of angle in the first direction so that the vehicle wheels rotatefurther in the first direction than requested by the driver. The motorcan reduce the offset when the driver toward center in a seconddirection by subtracting an predetermined amount of angle in the seconddirection so that the vehicle wheels rotate less far in the seconddirection than requested by the driver. A gear ratio can exist betweenthe motor angle and the superimposition angle, because the motor of theactive steering system is connected to the steering column via theharmonic gear.

During one of vehicle start-up, vehicle initialization, or modificationof certain vehicle settings, a difference between a setpoint value forthe motor angle and the actual motor angle can occur, and the activesteering system can mitigate the effect of steering gear ratio changeson the driver's perceived steering experience by governing actions bythe motor to reduce the difference between a setpoint value for themotor angle and the actual motor angle.

The present teachings further provide an active steering system for avehicle, comprising: (1) a difference module configured to calculate anoffset angle by subtracting a measured motor angle from a demanded motorangle; (2) a rate limitation module configured to reduce the offsetangle, the rate limitation module receiving a driver-selected steeringangle, a reference offset angle, and a delayed offset angle, andoutputting a modified offset angle; (3) an offset arbitration moduleconfigured to receive the offset angle from the difference module and ademanded motor angle, and to output a modified demanded motor angle anda delayed offset angle that can be reduced in a synchronized manner bythe rate limitation module as a function of at least one of a steeringwheel speed and an offset reduction rate; and (4) a state decisionmodule configured to receive the offset angle from the difference moduleand a synchronization request, and to output state information to theoffset arbitration module.

Certain objects and advantages of the present teachings will be setforth in part in the description which follows, and in part will beobvious from the description, or may be learned by practice of thepresent teachings. The objects and advantages of the teachings will berealized and attained by means of the elements and combinationsparticularly pointed out in the appended claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the present teachings, as claimed.

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentteachings and, together with the description, serve to explain theprinciples of the teachings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an exemplary embodiment of an activesteering system in accordance with the present teachings.

FIG. 2 shows a block diagram comprising modules of the active steeringsystem used to carry out an exemplary embodiment of a method inaccordance with the present teachings.

FIG. 3 is a graph illustrating operation of the rate limitation moduleof FIG. 2.

FIGS. 4A and 4B illustrate an example of a wheel steer angle that arisesduring execution of an algorithm in accordance with the presentteachings.

DESCRIPTION OF THE EMBODIMENTS

Reference will now be made in detail to embodiments of the presentteachings, examples of which are illustrated in the accompanyingdrawings.

An active steering system (also referred to as an active front axlesteering system or an active front steering (“AFS”) system) makes itpossible to modify a “steering” ratio of the steering wheel angle to thewheel steer angle (i.e., to the angle between a respective wheel and theroadway) by adding a superimposition angle. The superimposition anglecan be provided, for example, by an actuator having an electric motor.The actuator's electric motor itself has a motor angle. A gear ratioexists between the motor angle and the superimposition angle, becausethe motor of the active steering system is connected to the steeringcolumn via a gear, for example a harmonic gear. In an exemplaryembodiment, the motor angle is 50° and results in a 2° superimpositionangle.

In an active steering system in accordance with the present teachings,the desired wheel steer angle can be calculated by a suitable algorithm.Tuning of the algorithm can be modified while driving, for example whena comfort-oriented or sporty operating mode is selected by a driver.

During start-up, initialization, or modification (e.g., changing from acomfort-oriented to a sporty operating mode, or vice versa) of certainsettings, a difference between a setpoint value for the motor angle andthe actual motor angle can occur. The range of possible deviation cancomprise, for example, about 10° to about 360°. When a driver changes,for example, from a comfort mode to a sporty mode, the steering gearratio can change. If the steering gear ratio changes, the same steeringwheel angle will provide a different wheel steer angle or will changethe wheel steer angle. If measures are not taken to mitigate the effectof steering gear ratio changes on the driver's perceived steeringexperience, the actuator's motor attempts to set the setpoint value forthe motor angle with the electric motor at full speed. The consequenceof this is that the wheel steer angle is altered without the influenceof the driver and without taking into account the driver's intentionregarding driving or steering, which gives the driver the impressionthat the vehicle is going out of control.

FIG. 1 is a schematic diagram of an exemplary embodiment of an activesteering system, in which a method according to the present teachingscan be implemented. As shown, the active steering system comprises aharmonic drive 20 (also referred to as a differential or AFS gear)operatively connected to a steering wheel column for a steering wheel 10than can be actuated by a driver 5. A superimposition angle or motorangle produced by the harmonic drive 20 can be set via an electric motor25 (e.g., a hollow shaft motor), which sets a wheel steer angle of aleft wheel 11 and a right wheel 12 to a wheel steer angle δ_(rad)produced by the active steering system. The wheel steer angle δ_(rad)can be expressed as δ_(rad)=δ_(drv)+δ_(motor), where δ_(motor) is asuperimposition motor angle and δ_(drv) is a driver-selected steeringangle.

The harmonic drive 20 can therefore add or subtract a superimpositionangle or motor angle δ_(motor) to or from the driver-selected steeringangle δ_(drv). The sum of the motor angle δ_(motor) and thedriver-selected steering angle δ_(drv) acts on the steering gear, whichproduces the wheel steer.

δ_(drv) denotes a steering angle specified by the driver and δ_(motor)denotes the motor angle, which is produced by the electric motor 25 andis superimposed on the driver steering angle δ_(drv) via harmonic drive20. As shown in FIG. 1, a hydraulic or electric power assistance system(PAS) 50 can also be provided to assist the AFS motor 25 in steering theroad wheel.

The SOC block 30 detects an offset angle based on inputs such asδ_(drv), δ_(motor, meas), and δ_(motor, setpoint), and reduces theoffset angle by generating a motor angle δ_(motor, setpoint2) that isbased on inputs such as steering wheel speed and wheel steer angle, withthe rate of offset angle reduction being optionally based on, forexample, vehicle speed.

Offset angle, as used herein, equals the demanded motor angle(designated as δ_(motor) in FIG. 1) minus the measured motor angle(designated as δ_(motor, meas) in FIG. 1).

Setpoint value for the motor angle (designated as δ_(motor, setpoint2)in FIG. 1) equals the demanded motor angle (designated as δ_(motor) inFIG. 1) minus an angle of correction determined by the SOC block 30.

In certain embodiments of the present teachings, if there is nosynchronization request, the setpoint value for the motor angle(designated as δ_(motor, setpoint) in FIG. 1) is equal to the demandedmotor angle (designated as δ_(motor) in FIG. 1). If there is asynchronization request, the setpoint value for the motor angle(designated as δ_(motor, setpoint2) in FIG. 1) is equal to the demandedmotor angle (designated as δ_(motor) in FIG. 1) minus the offset anglemultiplied by a function F(W_SteWhl), where W_SteWhl is the steeringwheel speed and function F is thus dependent, inter alia, on thesteering wheel speed. F(W_SteWhl) can be a lookup table that sends out apercentage by which offset should be reduced. A resulting modifiedoffset y_(k) can therefore be equal to an offset angle multiplied by thevalue of function F(W_SteWhl).

Thus, if there is no synchronization request, the setpoint value isequal to the demanded motor angle, whereas, if there is asynchronization request, a factor dependent on the steering wheel speedand/or the wheel steer angle is included in the offset compensation.

FIG. 2 illustrates modules used to carry out a method of the presentteachings as set forth above. The modules include a rate limitationmodule 210 (also referred to as “rate limiter”), a state decision module220 (also referred to as “state decision”), an offset arbitration module230 (also referred to as “offset arbitration”) and a difference module240 (also referred to as “difference”).

The main task of the rate limitation module 210 is to reduce the offsetangle that occurs in the active steering system by analyzing adriver-selected steering angle δ_(drv), a reference offset angle valuex_(k), and a delayed offset angle value y_(k-1). The rate limitationmodule 210 outputs a modified offset angle value y_(k). The main task ofthe difference module 240 is to calculate the offset angle (which equalsthe demanded angle minus the measured angle). The offset angle value isinput into the state decision module 220 and the offset arbitrationmodule 230. The state decision module 220 can also receive asynchronization request and can output state information to the offsetarbitration module 230. The offset arbitration module 230 also receivesa demanded motor angle as input. The main task of the offset arbitrationmodule 230 is to output a newly-calculated setpoint value (either thedemanded motor angle unmodified or, in the case of a synchronizationrequest, a modified demanded motor angle in accordance with calculationsset forth above). The offset arbitration module 230 also outputs amodified offset angle value y_(k), which can be reduced in asynchronized manner by the rate limitation module 210 as a function ofthe steering wheel speed and the offset reduction rate. The offsetreduction rate R utilized by the rate limiter 210 can be calculated asfollows:

$R = \frac{x_{k} - y_{k - 1}}{t_{s}}$

with the boundary condition R_(low)<R<R_(high)

where x_(k) is the setpoint offset angle, t_(s) is the sample time,y_(k) is the current offset value, and y_(k-1) is a delayed offset value(e.g., delayed by one sample step).

The reference offset angle or setpoint offset angle value x_(k) is inputto the rate limitation module 210, and a modified offset angle valuey_(k) is output therefrom. The delayed offset angle value y_(k-1) is anoutput variable of the preceding step in the offset arbitration module230. The offset reduction rate R is then limited accordingly, and thecalculation for the next output step is carried out as follows:

y _(k) =t _(s) ·R _(high) +y _(k-1)

if R>R_(high)

y _(k) =t _(s) ·R _(low) +y _(k-1)

if R<R_(low)

y _(k) =x _(k)

if R is between R_(high) and R_(low)wherein t_(s)=t_(k)−t_(k-1) denotes a fixed sampling time.

The above equations are implemented to cover steering to the right andsteering to the left. R_(high) and R_(low) can be tuned by, for example,an engineer during an algorithm development phase.

FIG. 3 illustrates how the value of the current offset angle y_(k) canbe changed to achieve the setpoint value as x_(k) in accordance with theequations set forth above.

In the embodiment described above, the rate limitation algorithm wasdesigned in such a way that only the offset angle value was stored. Theequations set forth below illustrate that a rate limitation method inaccordance with the present teachings can have an offset angle valuey_(k) that is decoupled from the rate limitation method explained above.

If y_(k)=x_(k)−Δ_(k), then

$R = {\frac{x_{k} - y_{k - 1}}{t_{s}} = {\frac{x_{k} - \left( {x_{k - 1} - \Delta_{k - 1}} \right)}{t_{s}} = {\frac{x_{k} - x_{k - 1}}{t_{s}} + \frac{\Delta_{k - 1}}{t_{s}}}}}$

The output equations also change, with y being replaced by x and Δ sothat, if R>R_(high):

x _(k)−Δ_(k) =t _(s) ·R _(high) +x _(k-1)−Δ_(k-1).

From this it follows that:

Δ_(k)=Δ_(k-1) +x _(k) −x _(k-1) −t _(s) ·R _(high)

and if R<R_(low): x_(k)−Δ_(k)=t_(s)·R_(low)+x_(k-1)−Δ_(k-1) and itfollows that:

Δ _(k)=Δ_(k-1) +x _(k) −x _(k-1) −t _(s) ·R _(low)

If R_(low)<R<R_(high), then Δ_(k)=Δ_(k-1)

In certain embodiments, the reference offset or setpoint offset anglevalue x_(k) is always set to zero so that the offset can be brought downto zero over time as a function of steering wheel speed and vehiclespeed, i.e., so that the detected offset can be reduced to zero. Becausex_(k) is always set to zero, the following equations are obtained:

$\frac{x_{k} - \Delta_{k - 1}}{t_{s}} = \frac{x_{k} - \left( {x_{k - 1} - y_{k - 1}} \right)}{t_{s}}$$\frac{x_{k} - \Delta_{k - 1}}{t_{s}} = \frac{\left( {x_{k} - x_{k - 1}} \right) + y_{k - 1}}{t_{s}}$$\frac{x_{k} - \Delta_{k - 1}}{t_{s}} = \frac{y_{k - 1}}{t_{s}}$

The production of a new offset angle value thus can take the followingform:

Δ_(k)=Δ_(k-1) +t _(s) *R _(high)

if R>R _(high)

Δ_(k)=Δ_(k-1) +t _(s) *R _(low)

if R>R _(low)

Δ_(k) =x _(k)=0

if R _(low) <R<R _(high)

According to the above equations, the offset angle value is updated atevery sample step because the offset angle value is reduced at everytime step when possible. In accordance with certain embodiments of thepresent teachings, the algorithm above in the SOC 30 illustrated in FIG.1.

FIGS. 4A and 4B illustrate an example of how the wheel steer angle(SteWhl/8) can change when the driver selects a different mode ofsteering (e.g., from sport to comfort) and an associated variable gearratio (Angle VGR) changes.

FIG. 4A illustrates a jump in a desired road wheel angle (dotted line)caused by a change in steering ratio desired by the driver. But, thesteering wheel angle (SteWhl/8, reduced by a factor of eight in theplot) does not correspondingly change as desired. If an algorithm inaccordance with the present teachings was not utilized, the steeringwheel angle would have jumped with the dotted line.

The offset angle (i.e., the difference between the demanded motor angleand the measured motor angle) can be captured at the synchronizationrequest and set as the current offset angle to reduce the offset angle.The change in the motor angle, offset angle, and setpoint motor angleare plotted in FIG. 4B.

FIG. 4B illustrates, for the scenario of FIG. 4A, how the offset angleis reduced when the driver steers back to the center point. FIG. 4B alsoshows the correct capture and storing of offset angle value. The firstdip in the offset angle is due to system start-up and second dip is dueto driver selection of new steering mode. The offset angle value canremain unchanged as long as driver does not do any steering change. Whenthe driver starts to steer the offset angle can be reduced and a desiredroad steer angle can be achieved in a synchronised manner.

Other embodiments of the present teachings will be apparent to thoseskilled in the art from consideration of the specification and practiceof the teachings disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the present teachings being indicated by the following claims.

1. A method for operating an active steering system of a vehicle, in which a ratio between a wheel steer angle and a steering wheel angle specified by the driver can be modified by superimposition of a superimposition angle, the method comprising: detecting an offset between a requested motor angle and an actual motor angle; and reducing the offset between the requested motor angle and the actual motor angle using a reduction rate that is selected as a function of at least one input variable specified by the driver.
 2. The method of claim 1, wherein the at least one input variable specified by the driver comprises one or more of a steering wheel speed and a wheel steer angle.
 3. The method of claim 1, wherein reducing the offset comprises reducing the offset in a manner that is synchronized with a steering operation of the driver.
 4. The method of claim 1, wherein the superimposition angle is provided by a motor.
 5. The method of claim 1, wherein detecting the offset and reducing the offset occur only upon an external request for detection of the offset.
 6. The method of claim 1, wherein, in the absence of an external request for detection of the offset, a demanded superimposition angle is selected as a setpoint value for the superimposition angle. 7-18. (canceled) 